The present invention relates to output amplifiers of integrated circuits, and more particularly, an output amplifier in CMOS technology whose operating rate is likely to vary as a function of the environment parameters of its transistors (temperature, supply voltage and manufacturing quality).
An output amplifier is used for transmitting electric signals to the outside of a circuit. Generally, the signals are supplied to electric conductors (pins, tracks) which are deemed equivalent to inductive and capacitive loads. The function of the output amplifier is mainly to adapt the signal transmitted to the outside of the circuit to the electric line that receives the signal.
FIG. 1 shows in a diagram an output amplifier 10 which drives a generally capacitive load 12. The amplifier 10 includes a P-channel MOS switching transistor 14 connected between a supply voltage terminal Vdd and an output terminal 0, and controlled by the output of an inverter 16. An N-channel MOS switching transistor 18 is connected between ground and the output terminal 0 and is controlled by the output of an inverter 20. The inputs of the inverters 16 and 20 are together connected to an input terminal I.
The output amplifier is to produce a voltage signal having sufficient amplitude to be interpretable as a logic signal. With each transition the amplifier is to charge and discharge the capacitive load 12. The gradient of the transition depends on the current that the amplifier is able to produce and on the value of the capacitive load 12. If the current is insufficient and the operating frequency is too high, the gradient of the transition is too small for the required amplitude to be reached within a period of time.
The intrinsic conductivity of the transistors, thus the current that the transistors are able to produce, varies according to the circuit temperature, the value of the supply voltage and the manufacturing quality of the transistors, the latter depending on the manufacturing processes of the integrated circuit. When an output amplifier is designed, it is generally desired to guarantee that it operates at a predetermined frequency in a given temperature range and in a given supply voltage range, whatever the manufacturing quality of the transistors. This leads to designing the transistors so that they have the required conductivity in worst case conditions (high temperature, low supply voltage, poor-quality transistors). The real environment parameters of the transistors are never the worst parameters. As a result, the output amplifiers are capable of supplying higher currents than required, to such an extent that they can generate too much noise during the switching in some applications.
It is an object of the present invention to provide a device that enables to compensate the characteristic variations of an output amplifier caused by a variation of its environment parameters.
To achieve this object, the present invention provides a circuit whose operating rate varies as a function of temperature, supply voltage and intrinsic transistor quality of the circuit transistors, associated to a compensation circuit that includes a constant current source producing a current that is substantially constant and independent of the temperature, the supply voltage and the intrinsic quality of the transistors of the circuit, a variable current source producing a current that increases with the inverse of the temperature, the supply voltage and the intrinsic quality of the circuit transistors, and means for decreasing the operating rate of the circuit when the difference of the currents produced by the first and second sources increases.
According to an aspect of the present invention, said means are provided for decreasing the rate at which transistor control signals of the circuit vary when said current difference increases.
According to an aspect of the present invention, the circuit includes MOS a switching transistors connected in parallel, and said means are provided to concurrently turn on a decreasing number of the transistors when the current difference increases.
According to an aspect of the present invention, the circuit includes inverters connected in a ring that forms an oscillator, and said means are provided for increasing the number of inverters connected in the ring when said current difference increases.
According to an aspect of the present invention, the circuit includes a first MOS transistor of a first conductivity type connected between a first supply voltage and an output terminal, and an inverter having its output terminal connected to the gate of the transistor, the means for decreasing the rate including an adjustable current source connected between a second supply voltage and a supply terminal of the inverter, a second supply terminal of the inverter being connected to the first supply voltage.
According to an aspect of the present invention, the adjustable current source is a second MOS transistor of a second conductivity type, controlled by a voltage varying in the opposite direction to said current difference.
According to an aspect of the present invention, said current difference is a digital signal carried on several control lines, a decreasing number of which is activated for discrete increasing values of the difference, and the adjustable current source includes a group of MOS transistors of the second conductivity type connected in parallel, each of which is controlled by one of the control lines.
According to an aspect of the present invention, the variable current source includes a current mirror reproducing a current that flows through a second MOS transistor of the first conductivity type connected to the first supply voltage and whose gate is connected to the second supply voltage, and each control line is connected to an output of a current mirror reproducing a constant current and to an output of a current mirror connected to reproduce the current of the variable current source according to a predetermined ratio, different for each control line.
According to an aspect of the present invention, the gate of each switching transistor is connected to an output of a current mirror reproducing a constant current and to an output of the current mirror connected to reproduce the current of the variable current source according to a predetermined ratio, different for each control line.
According to an aspect of the present invention, said means generate a digital control signal carried on several control lines, a single line being activated at a time, the rank of the activated line increasing with said difference, the control lines being connected so that each line activates a loop including a number of inverters increasing with the rank of the line.